The present invention relates to a bead core used for a pneumatic tire and a pneumatic tire using the same, and more particularly, to a tire bead core and a pneumatic tire which can suppress torsion and deformation of the bead core to thereby improve driving stability.
Bead cores are embedded in bead portions of a pneumatic tire in order to ensure a fitting ability to a rim. Such bead cores are generally formed such that a cross-sectionally circular or oval bead wire covered with rubber is annularly wound and bundled in the form of a circle or hexagon in cross section, or is annularly wound and bundled, and further twisted in the form of a quadrangle in cross section.
The bead cores using the above cross-sectionally circular bead wire are such that bead wire portions next to each other contact at one point via covered rubber portions, and when a large lateral force is applied to a vehicle during cornering at a high speed and a large torsion force acts on the tires, the bead cores are distorted. As a result, the bead portions attached to the rim are distorted, thereby decreasing driving stability.
To solve the above problem, conventionally, for example, Unexamined Japanese Patent Application Publication Nos. 9-66712, 10-95210, and 49-119301 disclose bead wires with a quadrangular cross section such as a rectangular cross section used for a bead core of a pneumatic tire. The bead core comprises an annular body consisting of a plurality of winding portions radially stacked; the winding portions are formed from such a cross-sectionally quadrangular bead wire; each winding portion comprises sections of the bead wire annularly wound a plurality of times so as to align in a widthwise direction of the bead core in such a manner that the sections of the wound bead wire next to each other are in contact with each other. Since a contacting area between respective bead wire sections increases, the torsion of the bead core is suppressed, and driving stability is enhanced.
The above-mentioned bead core is, however, deformed in the cured tire; as a result, the bead core does not provide an effective tightening function contemplated in designing; the deformation of the bead core is one of the causes that deteriorates driving stability.